The present invention relates to a polymer matrix composite useful in thermal management applications and to a method for making such a composite.
Composite materials are well known for use in structural applications. In recent years, composites have also become desirable for use in thermal management applications. For example, composites have been found to be useful for heat dissipation in electronic packaging. Typical composites for electronic packaging components exhibit coefficients of thermal expansion, which may be tailored by varying the architecture of reinforcing agents. Such reinforcing agents may include conventional carbon fibers, silicon carbide particles, boron nitride particles, titanium nitride particles, and diamond particles. The resulting composite materials have not shown substantial improvements in thermal conductivity. More recently, vapor grown carbon fibers have been studied for use in thermal management applications. Such fibers are grown through the pyrolysis of hydrocarbon gas in the presence of a metal catalyst. Vapor grown carbon fibers have a higher thermal conductivity than any other carbon fiber and may be produced at a lower cost. Vapor grown carbon fibers also exhibit the highest degree of graphitic perfection of any known carbon fiber.
An aluminum matrix composite and a carbon matrix composite have been fabricated from this type of carbon fiber and exhibit high thermal conductivity, as taught in Ting, U.S. Pat. No. 5,814,408. However, these two composites are electrically conductive which is not desired in many applications. Furthermore, fabrication of the aluminum matrix composite requires a temperature higher than the aluminum solidus, which is about 660xc2x0 C. or 1220xc2x0 F., a high pressure to cast the molten aluminum, and a protective environment to prevent the oxidation of aluminum. Fabrication of the carbon matrix composite also requires a temperature higher than 1100xc2x0 C. or 2012xc2x0 F., and a reduced pressure. A prefrom is needed for making either the aluminum matrix composite or carbon matrix composite. Further, separate heating of matrix and preform is required for aluminum matrix composite and often required for carbon matrix composite. All these add to production costs.
Accordingly, there is still a need in the art for improved composites for use in thermal management applications which not only exhibit high thermal conductivity but also provide electrically insulating surfaces, a low density, and may be produced at lower cost.
It is an objective of the present invention to provide a polymer matrix composite having high thermal conductivity while having an insulating surface and low density.
It is another objective of the present invention to provide a method for preparing the polymer matrix composite perfectly suitable for use in thermal management applications.
The present invention meets that need for use in thermal management applications by providing a polymer matrix composite, which has high thermal conductivity, a low density, and an electrically insulating surface. The preparation of the polymer matrix composite requires only atmospheric environment and without the use of a preform. The polymer matrix composite of the invention may especially be used in a variety of thermal management applications where electric insulation and low density are preferable.
The polymer matrix composite of the present invention is formed from a stack of in-situ interwoven, graphitized vapor grown carbon fiber (VGCF) mats, eliminating the fabrication of a preform.
In accordance with one embodiment of the invention, a polymer matrix composite is provided which comprises at least one interwoven mat of graphitized vapor grown carbon fibers. The mat includes interstices therein which have been infiltrated with polymeric material.
Preferably, the mats are comprised of semi-aligned, semi-continuous vapor grown carbon fibers, which have been interwoven in situ during growth. The fibers are also preferably graphitized, i.e., they are heat treated at a temperature of about 2600xc2x0 C. or higher to convert a portion of the carbon contained in the fibers to graphitic carbon.
Preferably, the room temperature thermal conductivity of the vapor grown carbon fiber mat of the present invention is about 1500 W/m-K and above, which leads to a polymer matrix composite having a thermal conductivity above 400 W/m-K.
A method of making the polymer matrix composite is also provided which generally comprises the steps of providing at least one interwoven mat of graphitized vapor grown carbon fibers and infiltrating the interstices of the stack with liquidized polymeric material.
Preferably, the mat is infiltrated with polymeric material by a hot press technique in which both the polymeric material and the mat are heated at the same time to a temperature only slightly above the melting temperature of the polymeric material. When the temperature of the polymeric material and mat reaches the state, pressure is applied so that infiltration is initiated in the atmospheric environment.
The method of the present invention is cost-effective and efficient as the use of polymeric materials. The method eliminates the need of high temperature, preform, protective environment, reduced or high atmospheric pressure, and separate heating. In addition, the use of vapor grown carbon fiber mats eliminates the need to mechanically weave individual grown fibers into a mat prior to their use in the composite.
Such a hot press technique allows a useful range of fiber loading, i.e., from 10 volume percent to 60 volume percent, and either unidirectional or in-plane orthogonal (two-directional) fiber architecture without resulting in inhomogeneous infiltration. This enables the coefficient of thermal expansion and other properties of the composite to be controlled over a wider range and therefore increases the number of applications in which the resulting composites can be used.
In addition, the resulting polymer matrix composite exhibits a higher thermal conductivity than composites reinforced with any other carbon fibers.
Further, the resulting polymer matrix composite has a lower density than any other composite for use in thermal management.
The polymer matrix composite may be used in many thermal management applications including aircraft, spacecraft, electronic devices, and portable electronics.
Accordingly, it is a feature of the present invention to provide a polymer matrix composite formed from interwoven vapor grown carbon fiber mats, which exhibit high thermal conductivity, low density, and variable coefficient of thermal expansion. It is a further feature of the present invention to provide a cost-effective method of producing such a polymer matrix composite. These, and other features and advantages of the invention will become apparent to those skilled in the art after comprehending the following detailed description, the accompanying drawings, and the appended claims.